This invention relates generally to Night Vision Devices, and more specifically to an eyecup and optical filter for attachment to a vision aid device. The invention comprises an appliance for placing an optical filter between a Night Vision Device and the device user's eye. Several embodiments of such devices are disclosed and claimed in co-pending U.S. utility patent application Ser. No. 12/012,266 filed Jan. 31, 2008, which is incorporated herein by reference.
It is well known (e.g. Duplicity Theory as discussed in Graham's (Editor) Vision and Visual Perception (John Wiley and Sons, 1966)) that the human eye has two modes of function depending on the incident illumination levels. In this regard, the term “photopic” refers to the eye function at a relatively high level of illumination (daylight) and “scotopic” refers to function at relatively low light levels (night). Acuity and color sensitivity are significantly better for the photopic regime; sensitivity to low illumination is significantly better with scotopic vision.
An important fact is the time period required to transition from photopic to scotopic vision, a phenomenon termed dark adaptation time. Depending on prior conditions, this dark adaptation time can be 45 minutes or longer. During this transition, the eye and the associated vision system have difficulty forming images resulting in a condition called night blindness.
Night Vision Devices (NVDs) refer to a group of electronic devices used to augment the vision of the user in low light and/or dark conditions. There are at least four classes of such devices distinguished by the type of illumination amplified by their respective electronics. They are sometimes referred to as passive image intensifiers, infrared imaging devices, thermal imaging devices, and combinations of these called fusion devices. Other types of devices may include infrared sources to illuminate the scene.
These NVDs utilize a display to convert the electronic signals from the detectors into an image visible by the human eye. It is a well-known fact that such displays are sufficiently bright to cause the eye to function in the photopic regime or the intermediate regime between photopic and scotopic. Therefore when the user of the NVD removes the device or looks away from the display, he endures night blindness during the dark adaptation time.
Special filters such as that disclosed in U.S. Pat. No. 6,158,865, incorporated herein by reference, have been designed to shorten the dark adaptation time and reduce this night blindness. Other filters have been used to improve visual acuity.
The displays of the NVDs can also produce a general illumination of the user's face and/or eye socket. Often referred to as “signature” or “splash”, this is very undesirable because it will disclose the presence and location of the NVD user to hostile persons who happen to be looking in the direction of the NVD user.
Some previous designs have placed filters in the optical train of the NVD eyepiece objective before the final optic. This is undesirable because it alters the focal properties of the NVD eyepiece objective.